Former cabinet minister Sandra Pupatello will launch her bid Thursday in Toronto and Windsor, sources confirmed to The Canadian Press.
Citizenship and Immigration Minister Charles Sousa is also expected to throw his hat in the ring Saturday, when he makes a yet-unspecified announcement in Mississauga.
However, Sousa hasn't resigned his cabinet post yet — a prerequisite for launching a leadership bid.
Sousa, who holds the Toronto-area riding of Mississauga South, also serves as minister responsible for the 2015 Pan and Parapan American Games, with preparations for the event in full swing.
McGuinty wouldn't confirm the news, but had high praise for Pupatello, who served as his right hand while in Opposition.
"Sandra is both a lot of fun and a lot of force," he said Wednesday in Niagara-on-the-Lake after speaking at the annual Ontario Economic Summit.
"She's a force to be reckoned with. ... She brings a lot of experience, a lot of passion, and a lot of integrity to the race."
The premier didn't gush quite as much about Sousa, but said he'd welcome his entry into the race.
Pupatello also has an ally in Finance Minister Dwight Duncan, who vowed to support his longtime friend if she joined the leadership race.
She was elected in the riding of Windsor-Sandwich in 1995 and was elevated to cabinet in 2003 when the McGuinty Liberals first took office.
Pupatello took on various portfolios, from education to community and social services and economic development and trade.
She didn't run for re-election last year, saying she was retiring from politics. Liberal Teresa Piruzza now holds her former seat, which became Windsor West in 1999.
Both she and Sousa would be competing against Kathleen Wynne and Glen Murray — both Toronto MPPs — who resigned from cabinet to jump in the race earlier this week.
Murray, who was first elected in 2010, most recently served as minister of training, colleges and universities.
Wynne has held several posts, including education minister, since she was first elected in 2003. She's also received the endorsement of Labour Minister Linda Jeffrey, Attorney General John Gerretsen, Liberal MPPs David Zimmer and Mario Sergio and former cabinet minister Monique Smith, who left politics last year.
Gerard Kennedy, who lost to McGuinty by just 140 votes in 1996 and has held seats provincially and federally, is also rumoured to be seriously considering a leadership bid. All hopefuls have until Nov. 23 to take the plunge.
Meanwhile, Health Minister Deb Matthews crossed herself off the list of potential hopefuls.
She said she decided not to enter the race because she has more important work to do in her current job.
"It's not the time for a new health minister, in my opinion," Matthews said Wednesday in Niagara-on-the-Lake.
"I want to stay where I am, stay the course, accelerate the pace of change and continue to drive the change that improves health care."
McGuinty triggered the competition when he prorogued the legislature and announced his retirement on Oct. 15, saying he'll stay on until a new leader is chosen.
The minority Liberals may want a fresh start, but all the leadership candidates are tainted by the scandals that have plagued the government over the last year, said Progressive Conservative Vic Fedeli.
They include the costly cancellation of two gas plants, a criminal investigation into Ontario's Ornge air ambulance service and McGuinty's abrupt decision to shut down the legislature after announcing he'd step down.
Prorogation brought all legislative business to a standstill and killed planned committee hearings into the cancelled power plants in Oakville and Mississauga, as well as a rare contempt motion against Energy Minister Chris Bentley.
Critics point out that Wynne was campaign co-chair when the Liberals decided to move the Mississauga plant during last year's election campaign to save Liberal seats — including Sousa's.
All of them, including Pupatello, were at the cabinet table when decisions were made that allowed Ontario to sink into a "financial mess," saddled with a $14.4-billion deficit, a debt that's doubled and 600,000 people still unemployed, Fedeli said.
"They all sat at the table and nodded their heads approvingly," he said.
"It's the same crew that got us into the mess."
The Liberals will choose a new leader at a Toronto convention the weekend of Jan. 25, 2013.